There are far too many dogs allowed to roam freely off the lead where the owners lose sight of them and have no idea what the dog is up to. When the DCs were toodlers we had a fair few picnics gate crashed in this way. It's also a disturbance to ground nesting birds; dogs should be on the lead for this reason in the local nature reserve but it's widely ignored and I've even had one tosspot argue that we shouldn't have taken a child into the "dog walking woods" after his puppy approached my 8yo, jumped up and winded him making him cry.
There's frequently dogs ambling out of the nature reserve and on to the main road and the owners haven't a clue. I've also seen dogs roaming from the nearby field, into the nature reserve well ahead of their panting owners and trigger an awful cacophany after approaching a dog with leads warning that they're reactive.
I've never had an issue with a dog that clearly indicated that it's reactive, because I don't approach random dogs and antagonise them. Leads (particularly very long ones) don't solve all problems, but I've had much less bother with dogs on leads than off-lead dogs.
I do a lot of quiet trail running, and the glitch in the quiet places arguments is when you get jumped up on because the owner didn't expect someone else in the area. It doesn't take much to distract many dogs into ignoring their owner and ceasing to be under control. There's also wildlife, livestock and crops in such rural areas.
My dipshit owner of the week award goes to the one that let her dogs chase the front-runners at junior parkrun and failed to recall them the first or second time and continued to let them roam lead-less despite the 80 children running around the area. Could have been avoided with using a lead.
Leads are not a substitute for good training, but they do reduce the risk of harm to the dog, other people/ dogs and to the surrounding area and are a useful and often under-used tool in responsible dog management.